1. Field of the Invention PA1 2. Description of the Related Art
The present invention relates to a system and method for learning the transmission band of a multiplex packet transmitted through a trunk of a packet exchange or an asynchronous transmission mode exchange.
Referring to FIG. 1, there is shown a functional block diagram of a general packet exchange system, in which a terminal 110 belongs to a packet exchange 100 with which a packet exchange 100' is connected through a trunk 109, and a terminal 111 belongs to the packet exchange 100'.
Assume now that the terminal 110 generates a communication request directed to the terminal 111. In this case, prior to its communication, the terminal 110 informs a controller 101 provided in the exchange 100 of a call message via a call signal line 105. The controller 101, in response to the call message, secures the necessary communication route, communication channel, etc. between the terminals 110 and Ill and informs the adjacent exchange 100' of the call message through a call signal line 107 of the trunk 109 according to preset route information on different destinations, while performing various sorts of controls to be described later.
On the side of the adjacent exchange 100', a controller 101' in the exchange 100', when receiving the aforementioned call message, secures the necessary communication route and channel up to the destination terminal 111 and informs the terminal Ill of the call message through a call signal line 105'.
Thereafter, when it is desired to perform a packet communication between the terminals 110 and 111, the packet communication is carried out in accordance with the communication route set by the exchange 100 based on the call message received from the terminal 110.
More specifically, when the terminal 110 is sending a packet through a data line 106 to a trunk interface 103 for example, the exchange 100 functions to further transmit the packet from the trunk interface 103 through a trunk interface 103' of the opposing exchange 100' to the terminal 111. In this connection, if another packet is already being transmitted through the corresponding trunk, then the packet to be next transmitted is once stored in a memory 102-for later transmission after the trunk becomes not in use.
Shown in FIG. 2 is an exemplary format of the call message used in this sort of packet transmission control, and FIG. 3 shows the structure of a packet actually transmitted in the control.
In FIG. 2, the illustrated call message comprises a call number for identification in a call exchange; a message code indicative of the sort of the message such as, e.g., `01` (call setting), `02` (call acceptance) or `03` (call release)(in the case of the call acceptance and release, constituent data which will be explained become unnecessary); destination number data indicative of a destination exchange number and a destination terminal number; a seized-channel number seized as the corresponding terminal line or trunk according to the route data with respect to the destination number of the packet receiver; a medium code indicative of the medium type of the communication terminal as the call-message issuer such as, e.g., `01` (voice), `02` (data) or `03` (television conference terminal); a request packet rate (for example, 125 packets/sec.) required by the call message; and additional data such as other terminal communication attributes.
FIG. 4 shows a functional structure of the controller 101 of the prior art exchange for processing such a call message as containing these data elements. In operation of the controller 101, when a `call setting` message is supplied to a call message buffer 201 of the controller 101 in the exchange through the call signal line 105, this causes the buffer 201 to send a request packet rate to a subtracter 210 through a signal line 209. At the same time, the subtracter 210 also receives an output of a residual-trunk- capacity register 204 through a signal line 208. The output of the residual-trunk-capacity register 204 indicates the current residual trunk capacity value which is set as its initial value at a maximum packet rate value of associated one of trunks predetermined for each trunk and which is sequentially modified through such control as to be described later. The subtracter 210 performs a subtraction of the residual trunk capacity value minus the request packet rate value and applies its subtraction result to a comparator 203. The comparator 203 compares the received subtraction result with zero.
When the subtraction result is equal to or larger than zero, the comparator 203 applies the subtraction result through a signal line 207 to the residual-trunk- capacity register 204 to modify the residual trunk capacity value. The comparator 203 also informs the trunk interface 103 and the terminal interface 104 of a `call acceptance` message via a signal line 205, the call message buffer 201 and the other of the call signal line 105 for call registration.
On the other hand, when the aforementioned subtraction result value is smaller than zero, the comparator 203 cancels the subtraction result value and informs the terminal interface 104 of a `call release` message via a signal line 206, the call message buffer 201 and the call signal line 105 for call release. In this case, the residual trunk capacity value is kept at the previous value. Thereafter, if the call is accepted, then a packet communication is carried out between the terminals 110 and 111.
As has been already explained above, the exemplary format of the communication packet is expressed, as shown in FIG. 3, in terms of the channel number seized at the time of the call setting and communication data.
Using communication packets having such a format as mentioned above, packet communication is carried out between the terminal 110 and the exchange 100 and between the exchanges 100 and 1001. At this time, when the exchange 100 temporarily receives many packets from its terminals, the exchange 100 might not be able to immediately send them to the trunk 109. For the purpose of avoiding such a situation, such packets are temporarily stored in the memory 102 (refer to FIG. 1) of the exchange 100 so that as the trunk 109 becomes empty, the packets stored in the memory 102 are sent onto the trunk 109.
Shown in FIG. 5 is a transmission characteristic showing how to transmit each call generated based on such conventional control operation.
It will be clear from FIG. 5 that, in the prior art, the request packet rate at the stage of the call setting is set usually at the maximum transmission packet rate (in other words, transmission bearer rate), which results in that the request packet rate becomes extremely large when compared with the packet rate (measured packet rate) at which communication is actually effected. In other words, the transmission characteristic of the trunk through which such requested calls are multiplexed has such an idle band as shown in FIG. 5.
Such transmission characteristic adversely affects greatly a communication efficiency. For this reason, it has been impossible to realize such effect unique to the packet multiplex communication as obtained, e.g., when a call communication packet becomes temporarily idle, by transmitting another communication packet during the temporary idle period.
In this way, the prior art system of managing the transmission band of multiplex packets is arranged so that when a terminal issues a communication request, its residual trunk capacity is found usually on the basis of the request packet rate given as the maximum packet rate of the associated terminal so that a call is accepted during zero or more of the residual trunk capacity. As a result, the prior art system has had such a problem that not only the request trunk capacity per call becomes much larger than the actual packet communication amount (traffic) of the trunk per call in the actual communication but also the packet multiplex effect of the trunk cannot be obtained, thus making it impossible to manage and operate the trunk transmission band efficiently.